In oil and gas producing operations, as produced fluid is delivered to the surface from the well, the fluid must be treated at the site to separate the crude oil and natural gas from the other fluids prior to their entry into the pipeline system. Basic sediment and water, which are initially present in the produced fluid in varied quantities, are considered by-products and impurities in produced petroleum. In addition to containing basic sediment and water, recovered petroleum is a complex mixture of many different compounds of hydrocarbons, all with different densities, pressures and characteristics which vary from well to well. This is further complicated as hydrocarbons are mixed in varied combinations with other fluids and sediment. As a result, the produced fluid from each well will have its own characteristics which determine the specific type of treating facilities that will be required to remove any by-products and impurities.
FIG. 1 illustrates the different types of equipment used in a representative field processing system. The produced fluid from the wellhead 1 flows into a separator 2 for the first stage of separation. These horizontal or vertical separators 2 are used to separate gases from liquids. Additional first stage separator units include freewater knockout units and settling tanks, which take advantage of the weight differences of oil, water and other impurities in the produced fluid to facilitate their separation. A significant portion of the gas, water, and sediment will be removed from the produced crude using first stage separator units. The second stage of separation will often use some type of heater-treater 3 to break emulsions of water and oil. After treatment, the produced fluid will be stored in units 4, and the wastewater which was separated from the produced fluid will eventually be disposed of.
The disposal of the produced wastewater should be done in an economically feasible manner consistent with current environmental regulations, with consideration given to the particular.type and location of the operation. To meet current environmental regulation requirements on acceptable hydrocarbon levels in discharged wastewater, it may be necessary to treat produced wastewater (as well as any other wastewater, such as rain water, wash-down water, and drain water) to lower its hydrocarbon content below that normally obtained from the crude-oil separation process described above. Another important incentive for lowering hydrocarbon concentration in produced wastewater is that the wastewater, in many cases, will need to be reinjected back into the reservoir for disposal, for maintaining reservoir pressure, or for increasing recovery through water flooding. To avoid plugging and permeability reduction, the dispersed oil concentration in the produced wastewater should typically not exceed about 50 mg/l.
The basic technical challenge in separating hydrocarbons from produced wastewater is how to economically separate the micron size oil droplets and/or oil-in-water emulsions (collectively referred to hereinafter as "droplets") from the water continuous phase. These very small droplets result in significant part from the severe shearing that occurs as pressure is reduced from the wellhead through producing equipment on the surface. The severe turbulence, aggravated by gas breaking out of the solution, results in very small oil droplets or oil-in-water emulsions being left in the produced wastewater. This situation can be aggravated further when centrifugal pumps are used to move the wastewater between process vessels. Valves used to control flows can also cause shearing of droplets into smaller ones, thus making removal of the oil droplets from the produced wastewater even more difficult.
Typical primary treatment processes for the removal of oil and other impurities from produced wastewater utilize standard separation equipment such as skim tanks, skim vessels, and corrugated plate interceptors (CPI's). However, these primary treatment processes will only remove oil droplets which are about 20 microns or larger in diameter. Depending on the severity of the problem, secondary treatment using CPI's (if none were used in the primary treatment), gas floatation units, chemically-assisted flotation units, hydrocyclones, matrix/grid type coalescers, multi-media filtration processes, centrifuges and biotreatment processes may also be needed to reduce oil concentration to allowable discharge limits. Depending on which type of treatment is used, oil droplets as small as 5-10 microns may be removed.
Each of these secondary treatment processes have problems associated with their use, and none of them are cost effective or simple to operate. For example, flotation units generally do not handle rate surges or oil slugs well and they, as with centrifuges, entail high capital and operator costs along with a high degree of operator skill. Biotreatment processes are also expensive due to the cost of the micro-organisms required and the associated storage problems. Additionally, none of these secondary processes are flexible because, to be cost effective, the wastewater will be pretreated using the primary equipment described above to ensure that the larger (20 microns or larger) oil droplets are removed prior to further processing with secondary equipment. For example, prior to utilizing a centrifuge, the wastewater will typically be treated with a skim tank or a CPI in order to efficiently separate the oil from the wastewater. This is done because, with most secondary treatment processes, only a fraction of the entire wastewater stream can be processed at a time. Thus, processing without prior primary treatment would be inefficient and expensive.